Here’s an interesting finding. Chasteberry (Vitex agnus-castus) is said to bind dopamine 2 (D2) receptors. Why is that interesting? Because so does apomorphine, an ED drug.

But it gets even weirder. A low dose of chasteberry binds to D2 receptors – killing desire. But high levels of chasteberry increase desire – by reducing prolactin, the evil chemical that creates the refractory period after orgasm.

But which dose is too little and which is too much?

As it says on Wikipedia (http://en.wikipedia.org/wiki/Vitex_agnus-castus):

The mechanism of action [of chastberry] is not fully understood[19] but it is assumed that it has dopaminergic effects resulting in changes of prolactin secretion. At low doses, such as might have been used in previous centuries for suppression of sexual desire, it inhibits activation of dopamine 2 receptor by competitive binding, causing a slight increase in release of prolactin. In higher concentrations, as in modern extracts, the binding activity is sufficient to reduce the release of prolactin. A study has found that treatment of 20 healthy men with higher doses of Vitex agnus-castus was associated with a slight reduction of prolactin levels, whereas lower doses caused a slight increase as compared to doses of placebo.[20] A decrease of prolactin will influence levels of Follicle-stimulating hormone (FSH) and estrogen in women ;[citation needed] and testosterone in men .[citation needed] Dopaminergic compounds (diterpenes with prolactin-suppressive effects that were almost identical in their prolactin-suppressive properties than dopamine itself) present in Vitex agnus castus seem likely to be the clinically important compounds which improve premenstrual mastodynia and possibly also psycho-somatic symptoms of PMS.[21]

Here’s a study that offers a high dose of 480 mg for lowering prolactin:
http://www.ncbi.nlm.nih.gov/pubmed/9021345

From the study:

The effects of three doses of a special Agnus castus extract (BP1095E1)–extracts from 120 mg, 240 mg and 480 mg of drug per day–were examined within the framework of a placebo-controlled clinical study of tolerance and prolactin secretion in 20 healthy male subjects during a period of 14 days. There was good tolerance during the study as regards the following: adverse effects, the effects on blood pressure and heart rate, blood count, Quick’s test, clinical chemistry as well as testosterone, FSH and LH values. During each study phase the 24-hour prolactin secretion profile was measured from the penultimate to the final day, and the amount of prolactin release was monitored an hour after TRH stimulation on the last day. A significant increase in the 24-hour profile was registered with the lowest dose in comparison to placebo, the opposite being the case with the higher doses, i.e. a slight reduction. In contrast to the administration of placebo, the 1-hour AUC after TRH stimulation resulted in a significant increase with the lowest dose and a significant reduction with the highest dose. The results suggest effects of the special Agnus castus extract which are dependent on the dose administered and the initial level of prolactin concentration.